It seems when you spend all day around computers, the obvious answer is always too easy.
Here's what I've done, and it appears to have resolved the problem. Thanks to Jolancer's advice.
I shut down the machine and opened up the hood. Pressed all cables in tight and firm. Did it a second time. Removed the hard drive and then replaced it, this time using a little extra force to make sure it was connected.
Booted the machine and took a quick look at CMOS and BIOS. Enabled SMART on the HD, saved and exited.
Continued the boot. Windows loaded without fail, Hard drive is purring along. I feel pretty foolish for overlooking the bleeding obvious!
Anyway, thanks guys for all the help. Thanks especially to Jolancer.
Next step - format and install Fedora Linux (I'll let you know how I go!).